Serum Integrated Screen
The Serum Integrated Screen is a two-part screening test that assesses the risk of a baby being born with Down syndrome, trisomy 18, and open neural tube defects (ONTDs). The risk for Smith-Lemli-Optiz syndrome (SLOS) is reported if the patient is high risk. The first part can be performed between 11 weeks and 13 weeks, 6 days of gestation. The second part can be performed between 15 and 22 weeks gestation.
In the first part, a maternal blood sample is drawn to measure the level of one serum analyte:
- Pregnancy-associated plasma protein (PAPP-A)
In the second part, a second maternal blood sample is taken to measure the levels of four serum analytes:
- Alpha-fetoprotein (AFP)
- Total beta-human chorionic gonadotropin (total ß-hCG)
- Unconjugated estriol (uE3)
In pregnancies affected with Down syndrome, the levels of PAPP-A, AFP, and uE3 tend to be lower than normal while the levels of inhibin and total ß-hCG are elevated.
In pregnancies affected with trisomy 18, the levels of PAPP-A, AFP, uE3, inhibin-A, and ß-hCG tend to be lower than normal.
In pregnancies affected with ONTDs such as open spina bifida or anencephaly, the level of AFP tends to be elevated.
What Does a Negative Result Mean?
The result is considered screen negative if the risks for Down syndrome, trisomy 18 and ONTDs are lower than the cutoff used. Typically, a cutoff of one in 270 is used for Down syndrome, a cutoff of one in 100 is used for trisomy 18, and a cutoff of one in 104 is used for ONTDs. A patient with a screen negative result is not considered at high risk to have a baby with either Down syndrome or trisomy 18. However, the chance to have a pregnancy with Down syndrome or trisomy 18 is not completely eliminated.
What Does a Positive Result Mean?
A screen positive result for Down syndrome by the Serum Integrated Screen means that the patient has a risk to have a baby with Down syndrome is greater than or equal to one in 270. About 5% of patients who have the Serum Integrated Screen receive a screen positive result. A screen positive result for trisomy 18 means that the patient’s risk of having a baby with trisomy 18 is greater than or equal to one in 100 after both parts. A screen positive result for ONTDs means that the patient’s risk of having a baby with an ONTD is greater than or equal to one in 104.
A high-risk result for Smith-Lemli-Optiz syndrome (SLOS) will be reported if the patient has a risk of greater than or equal to one in 50. No risk assessment will be provided for this condition if the patient is low risk.
What is the Detection Rate for Down Syndrome, Trisomy 18 and ONTDs?
The Serum Integrated Screen will detect about 92% of singleton pregnancies affected with Down syndrome, 90% of singleton pregnancies affected with trisomy 18, and 85% of singleton pregnancies affected with ONTDs.
What Other Tests may be Offered if a Serum Integrated Screen is Positive?
Depending on the results of the Serum Integrated Screen, additional testing may be offered including a second trimester detailed anatomy ultrasound and prenatal diagnosis (amniocentesis). Genetic counseling is also recommended.